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<tr class="t-nv"><td colspan="5"> <a href="attributes.html" title="cpp/language/attributes"> Attributes</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Types</td></tr>
<tr class="t-nv"><td colspan="5"> <a href="typedef.html" title="cpp/language/typedef"> <code>typedef</code> declaration</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="type_alias.html" title="cpp/language/type alias"> Type alias declaration</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Casts</td></tr>
<tr class="t-nv"><td colspan="5"> <a href="implicit_cast.html" title="cpp/language/implicit conversion"> Implicit conversions</a> - <a href="explicit_cast.html" title="cpp/language/explicit cast"> Explicit conversions</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="static_cast.html" title="cpp/language/static cast"><tt>static_cast</tt></a> - <a href="dynamic_cast.html" title="cpp/language/dynamic cast"><tt>dynamic_cast</tt></a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="const_cast.html" title="cpp/language/const cast"><tt>const_cast</tt></a> - <a href="reinterpret_cast.html" title="cpp/language/reinterpret cast"><tt>reinterpret_cast</tt></a></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Memory allocation</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="new.html" title="cpp/language/new"> <code>new</code> expression</a></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="delete.html" title="cpp/language/delete"> <code>delete</code> expression</a></td></tr>
</table></div></td></tr>
<tr class="t-nv-h1"><td colspan="5"> <a href="classes.html" title="cpp/language/classes"> Classes</a></td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="class.html" title="cpp/language/class"> Class declaration</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="initializer_list.html" title="cpp/language/constructor"> Constructors</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="this.html" title="cpp/language/this"> <code>this</code> pointer</a></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="access.html" title="cpp/language/access"> Access specifiers</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="friend.html" title="cpp/language/friend"> <code>friend</code> specifier</a></td></tr>
</table></div></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Class-specific function properties</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="virtual.html" title="cpp/language/virtual"> Virtual function</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="override.html" title="cpp/language/override"> <code>override</code> specifier</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="final.html" title="cpp/language/final"> <code>final</code> specifier</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="explicit.html" title="cpp/language/explicit"><tt>explicit</tt></a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="static.html" title="cpp/language/static"><tt>static</tt></a> </td></tr>
</table></div></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Special member functions</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="default_constructor.html" title="cpp/language/default constructor"> Default constructor</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="copy_constructor.html" title="cpp/language/copy constructor"> Copy constructor</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="move_constructor.html" title="cpp/language/move constructor"> Move constructor</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="as_operator.html" title="cpp/language/as operator" class="mw-redirect"> Copy assignment</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="move_operator.html" title="cpp/language/move operator" class="mw-redirect"> Move assignment</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="destructor.html" title="cpp/language/destructor"> Destructor</a></td></tr>
</table></div></td></tr>
<tr class="t-nv-h1"><td colspan="5"><a href="templates.html" title="cpp/language/templates"> Templates</a> </td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="class_template.html" title="cpp/language/class template"> Class template </a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="function_template.html" title="cpp/language/function template"> Function template </a></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="template_specialization.html" title="cpp/language/template specialization"> Template specialization</a></td></tr>   
<tr class="t-nv"><td colspan="5"> <a href="parameter_pack.html" title="cpp/language/parameter pack"> Parameter packs</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
</table></div></td></tr>
<tr class="t-nv-h1"><td colspan="5"> Miscellaneous </td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="asm.html" title="cpp/language/asm"> Inline assembly</a></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="history.html" title="cpp/language/history"> History of C++</a></td></tr>
</table></div></td></tr>
</table></div>
</div><div></div></div></div></div><div class="t-navbar-sep"> </div><div class="t-navbar-head"><a href="functions.html" title="cpp/language/functions"> Functions</a><div class="t-navbar-menu"><div><div style="display:inline-block">
<div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv-h2"><td colspan="5"> Declarations </td></tr>
<tr class="t-nv"><td colspan="5"> <strong class="selflink"> function declaration</strong> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="function.html#Parameter_list" title="cpp/language/function"> function parameter list</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="function.html#Function_definition" title="cpp/language/function"> function definition</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="default_arguments.html" title="cpp/language/default arguments"> default arguments</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="variadic_arguments.html" title="cpp/language/variadic arguments"> variadic arguments</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="inline.html" title="cpp/language/inline"> inline specifier</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="lambda.html" title="cpp/language/lambda"> lambda expression</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="coroutines.html" title="cpp/language/coroutines"> coroutines </a> <span class="t-mark-rev t-since-cxx20">(C++20)</span></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Function calls </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="adl.html" title="cpp/language/adl"> argument-dependent lookup (ADL)</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="operator_other.html#Built-in_function_call_operator" title="cpp/language/operator other"> function-call operator</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../named_req/FunctionObject.html" title="cpp/named req/FunctionObject"> function objects</a> </td></tr>
<tr class="t-nv-h2"><td colspan="5"> Overloading </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="overload_resolution.html" title="cpp/language/overload resolution"> overload resolution</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="operators.html" title="cpp/language/operators"> operator overloading</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="overloaded_address.html" title="cpp/language/overloaded address"> address of an overload set</a> </td></tr>
</table></div>
</div><div></div></div></div></div><div class="t-navbar-sep"> </div></div>
<p>A function declaration introduces the function name and its type. A function definition associates the function name/type with the function body. 
</p>
<h3><span class="mw-headline" id="Function_declaration">Function declaration</span></h3>
<p>Function declarations may appear in any scope. A function declaration at class scope introduces a class member function (unless the friend specifier is used), see <a href="member_functions.html" title="cpp/language/member functions">member functions</a> and <a href="friend.html" title="cpp/language/friend">friend functions</a> for details.
</p><p>The type of the function being declared is composed from the <i>return type</i> (provided by the <span class="t-spar">decl-specifier-seq</span> of the <a href="declarations.html" title="cpp/language/declarations">declaration syntax</a>) and the function <span class="t-spar">declarator</span>
</p>
<table class="t-sdsc-begin">

<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">noptr-declarator</span> <code><b>(</b></code> <span class="t-spar">parameter-list</span> <code><b>)</b></code> <span class="t-spar">cv</span><span class="t-mark">(optional)</span> <span class="t-spar">ref</span><span class="t-mark">(optional)</span> <span class="t-spar">except</span><span class="t-mark">(optional)</span> <span class="t-spar">attr</span><span class="t-mark">(optional)</span>
</td>
<td> (1)
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">noptr-declarator</span> <code><b>(</b></code> <span class="t-spar">parameter-list</span> <code><b>)</b></code> <span class="t-spar">cv</span><span class="t-mark">(optional)</span> <span class="t-spar">ref</span><span class="t-mark">(optional)</span> <span class="t-spar">except</span><span class="t-mark">(optional)</span> <span class="t-spar">attr</span><span class="t-mark">(optional)</span> <code><b>-&gt;</b></code> <span class="t-spar">trailing</span>
</td>
<td> (2)
</td>
<td> <span class="t-mark-rev t-since-cxx11">(since C++11)</span>
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<p>(see <a href="declarations.html" title="cpp/language/declarations">Declarations</a> for the other forms of the <span class="t-spar">declarator</span> syntax)
</p>
<div class="t-li1"><span class="t-li">1)</span> Regular function declarator syntax</div>
<div class="t-li1"><span class="t-li">2)</span> Trailing return type declaration: trailing return type is only allowed on the outermost function declarator. The decl-specifier-seq in this case must contain the keyword <code>auto</code> </div>
<table class="t-par-begin">


<tr class="t-par">
<td> <span class="t-spar">noptr-declarator</span>
</td>
<td> -
</td>
<td> any valid <span class="t-spar">declarator</span>, but if it begins with *, &amp;, or &amp;&amp;, it has to be surrounded by parentheses.
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">parameter-list</span>
</td>
<td> -
</td>
<td> possibly empty, comma-separated list of the function parameters (see below for details)
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">attr</span>
</td>
<td> -
</td>
<td> <span class="t-mark-rev t-since-cxx11">(since C++11)</span> optional list of <a href="attributes.html" title="cpp/language/attributes">attributes</a>. These attributes are applied to the type of the function, not the function itself. The attributes for the function appear after the identifier within the declarator and are combined with the attributes that appear in the beginning of the declaration, if any.
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">cv</span>
</td>
<td> -
</td>
<td> const/volatile qualification, only allowed in non-static member function declarations
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">ref</span>
</td>
<td> -
</td>
<td> <span class="t-mark-rev t-since-cxx11">(since C++11)</span> ref-qualification, only allowed in non-static member function declarations
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">except</span>
</td>
<td> -
</td>
<td>  <table class="t-rev-begin">
<tr class="t-rev t-until-cxx11"><td>
<p><a href="except_spec.html" title="cpp/language/except spec">dynamic exception specification</a>
</p>
</td>
<td><span class="t-mark-rev t-until-cxx11">(until C++11)</span></td></tr>
<tr class="t-rev t-since-cxx11 t-until-cxx17"><td>
<p>either <a href="except_spec.html" title="cpp/language/except spec">dynamic exception specification</a><br>or <a href="noexcept_spec.html" title="cpp/language/noexcept spec">noexcept specification</a>
</p>
</td>
<td><span class="t-mark-rev t-since-cxx11">(since C++11)</span><br><span class="t-mark-rev t-until-cxx17">(until C++17)</span></td></tr>
<tr class="t-rev t-since-cxx17"><td>
<p><a href="noexcept_spec.html" title="cpp/language/noexcept spec">noexcept specification</a>
</p>
</td>
<td><span class="t-mark-rev t-since-cxx17">(since C++17)</span></td></tr>
</table>
 <table class="t-rev-begin">
<tr class="t-rev t-until-cxx17"><td>
<p>Note that the exception specification is not part of the function type
</p>
</td>
<td><span class="t-mark-rev t-until-cxx17">(until C++17)</span></td></tr>
</table>
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">trailing</span>
</td>
<td> -
</td>
<td> Trailing return type, useful if the return type depends on argument names, such as <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T, <span class="kw1">class</span> U<span class="sy1">&gt;</span> <span class="kw4">auto</span> add<span class="br0">(</span>T t, U u<span class="br0">)</span> <span class="sy2">-</span><span class="sy1">&gt;</span> decltype<span class="br0">(</span>t <span class="sy2">+</span> u<span class="br0">)</span><span class="sy4">;</span></span></span> or is complicated, such as in <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">auto</span> fpif<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy2">-</span><span class="sy1">&gt;</span><span class="kw4">int</span><span class="br0">(</span><span class="sy2">*</span><span class="br0">)</span><span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span></span></span>
</td></tr></table>
<p><br>
</p>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx20"><td>
<p>As mentioned in <a href="declarations.html#Declarators" title="cpp/language/declarations">Declarations</a>, the declarator can be followed by a <i>requires</i> clause, which declares the associated <a href="constraints.html" title="cpp/language/constraints">constraints</a> for the function, which must be satisfied in order for the function to be selected by <a href="overload_resolution.html" title="cpp/language/overload resolution">overload resolution</a>. (example: <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span> f1<span class="br0">(</span><span class="kw4">int</span> a<span class="br0">)</span> requires <span class="kw2">true</span><span class="sy4">;</span></span></span>) Note that the associated constraint is part of function signature, but not part of function type.
</p>
</td>
<td><span class="t-mark-rev t-since-cxx20">(since C++20)</span></td></tr>
</table>
<p>Function declarators can be mixed with other declarators, where decl-specifier-seq allows:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co1">// declares an int, an int*, a function, and a pointer to a function</span>
<span class="kw4">int</span> a <span class="sy1">=</span> <span class="nu0">1</span>, <span class="sy2">*</span>p <span class="sy1">=</span> <a href="../types/NULL.html"><span class="kw109">NULL</span></a>, f<span class="br0">(</span><span class="br0">)</span>, <span class="br0">(</span><span class="sy2">*</span>pf<span class="br0">)</span><span class="br0">(</span><span class="kw4">double</span><span class="br0">)</span><span class="sy4">;</span>
<span class="co1">// decl-specifier-seq is int</span>
<span class="co1">// declarator f() declares (but doesn't define)</span>
<span class="co1">//                a function taking no arguments and returning int</span>
 
<span class="kw1">struct</span> S
<span class="br0">{</span>
    <span class="kw1">virtual</span> <span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">char</span><span class="br0">)</span> <span class="kw4">const</span>, g<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span> <span class="sy3">&amp;&amp;</span><span class="sy4">;</span> <span class="co1">// declares two non-static member functions</span>
    <span class="kw1">virtual</span> <span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">char</span><span class="br0">)</span>, x<span class="sy4">;</span> <span class="co1">// compile-time error: virtual (in decl-specifier-seq)</span>
                            <span class="co1">// is only allowed in declarations of non-static</span>
                            <span class="co1">// member functions</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx20"><td>
<p>Using a volatile-qualified object type as parameter type or return type is deprecated.
</p>
</td>
<td><span class="t-mark-rev t-since-cxx20">(since C++20)</span></td></tr>
</table>
<p>The return type of a function cannot be a function type or an array type (but can be a pointer or reference to those).
</p>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx11"><td>
<p>As with any declaration, attributes that appear before the declaration and the attributes that appear immediately after the identifier within the declarator both apply to the entity being declared or defined (in this case, to the function):
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="br0">[</span><span class="br0">[</span>noreturn<span class="br0">]</span><span class="br0">]</span> <span class="kw4">void</span> f <span class="br0">[</span><span class="br0">[</span>noreturn<span class="br0">]</span><span class="br0">]</span> <span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// okay: both attributes apply to the function f</span></pre></div></div>
<p>However, the attributes that appear after the declarator (in the syntax above), apply to the type of the function, not to the function itself:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span> <span class="br0">[</span><span class="br0">[</span>noreturn<span class="br0">]</span><span class="br0">]</span><span class="sy4">;</span> <span class="co1">// error: this attribute has no effect on the function itself</span></pre></div></div>
</td>
<td><span class="t-mark-rev t-since-cxx11">(since C++11)</span></td></tr>
</table>
<p>As with any declaration, the type of the function <code>func</code> declared as <code>ret func(params)</code> is <code>ret(params)</code> (except for parameter type rewriting described below): see <a href="type-id.html#Type_naming" title="cpp/language/type">type naming</a>.
</p>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx14"><td>
<h3> <span class="mw-headline" id="Return_type_deduction">Return type deduction</span></h3>
<p>If the <i>decl-specifier-seq</i> of the function declaration contains the keyword <code>auto</code>, trailing return type may be omitted, and will be deduced by the compiler from the type of the expression used in the <a href="return.html" title="cpp/language/return">return</a> statement. If the return type does not use <code>decltype(auto)</code>, the deduction follows the rules of <a href="template_argument_deduction.html#Other_contexts" title="cpp/language/template argument deduction">template argument deduction</a>:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">int</span> x <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span>
<span class="kw4">auto</span> f<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> x<span class="sy4">;</span> <span class="br0">}</span>        <span class="co1">// return type is int</span>
<span class="kw4">const</span> <span class="kw4">auto</span><span class="sy3">&amp;</span> f<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> x<span class="sy4">;</span> <span class="br0">}</span> <span class="co1">// return type is const int&amp;</span></pre></div></div>
<p>If the return type is <code>decltype(auto)</code>, the return type is as what would be obtained if the expression used in the return statement were wrapped in <a href="decltype.html" title="cpp/language/decltype">decltype</a>:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">int</span> x <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span>
decltype<span class="br0">(</span><span class="kw4">auto</span><span class="br0">)</span> f<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> x<span class="sy4">;</span> <span class="br0">}</span>  <span class="co1">// return type is int, same as decltype(x)</span>
decltype<span class="br0">(</span><span class="kw4">auto</span><span class="br0">)</span> f<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span><span class="br0">(</span>x<span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span> <span class="co1">// return type is int&amp;, same as decltype((x))</span></pre></div></div>
<p>(note: "<code>const decltype(auto)&amp;</code>" is an error, <code>decltype(auto)</code> must be used on its own)
</p><p>If there are multiple return statements, they must all deduce to the same type:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">auto</span> f<span class="br0">(</span><span class="kw4">bool</span> val<span class="br0">)</span>
<span class="br0">{</span>
    <span class="kw1">if</span> <span class="br0">(</span>val<span class="br0">)</span> <span class="kw1">return</span> <span class="nu0">123</span><span class="sy4">;</span> <span class="co1">// deduces return type int</span>
    <span class="kw1">else</span> <span class="kw1">return</span> <span class="nu17">3.14f</span><span class="sy4">;</span>   <span class="co1">// error: deduces return type float</span>
<span class="br0">}</span></pre></div></div>
<p>If there is no return statement or if the argument of the return statement is a void expression, the declared return type must be either <span class="t-c"><span class="mw-geshi cpp source-cpp">decltype<span class="br0">(</span><span class="kw4">auto</span><span class="br0">)</span></span></span>, in which case the deduced return type is <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span></span></span>, or (possibly cv-qualified) <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">auto</span></span></span>, in which case the deduced return type is then (identically cv-qualified) <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span></span></span>:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">auto</span> f<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>              <span class="co1">// returns void</span>
<span class="kw4">auto</span> g<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span> <span class="co1">// returns void</span>
<span class="kw4">auto</span><span class="sy2">*</span> x<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>             <span class="co1">// error: cannot deduce auto* from void</span></pre></div></div>
<p>Once a return statement has been seen in a function, the return type deduced from that statement can be used in the rest of the function, including in other return statements:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">auto</span> sum<span class="br0">(</span><span class="kw4">int</span> i<span class="br0">)</span>
<span class="br0">{</span>
    <span class="kw1">if</span> <span class="br0">(</span>i <span class="sy1">==</span> <span class="nu0">1</span><span class="br0">)</span>
        <span class="kw1">return</span> i<span class="sy4">;</span>              <span class="co1">// sum’s return type is int</span>
    <span class="kw1">else</span>
        <span class="kw1">return</span> sum<span class="br0">(</span>i <span class="sy2">-</span> <span class="nu0">1</span><span class="br0">)</span> <span class="sy2">+</span> i<span class="sy4">;</span> <span class="co1">// okay: sum’s return type is already known</span>
<span class="br0">}</span></pre></div></div>
<p>If the return statement uses a brace-init-list, deduction is not allowed:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">auto</span> func <span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="br0">{</span><span class="nu0">1</span>, <span class="nu0">2</span>, <span class="nu0">3</span><span class="br0">}</span><span class="sy4">;</span> <span class="br0">}</span> <span class="co1">// error</span></pre></div></div>
<p><a href="virtual.html" title="cpp/language/virtual">Virtual functions</a> <span class="t-rev-inl t-since-cxx20"><span>and <a href="coroutines.html" title="cpp/language/coroutines">coroutines</a> </span> <span><span class="t-mark-rev t-since-cxx20">(since C++20)</span></span></span>cannot use return type deduction:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> F
<span class="br0">{</span>
    <span class="kw1">virtual</span> <span class="kw4">auto</span> f<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="nu0">2</span><span class="sy4">;</span> <span class="br0">}</span> <span class="co1">// error</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
<p><a href="function_template.html" title="cpp/language/function template">Function templates</a> other than <a href="cast_operator.html" title="cpp/language/cast operator">user-defined conversion functions</a> can use return type deduction. The deduction takes place at instantiation even if the expression in the return statement is not <a href="dependent_name.html" title="cpp/language/dependent name">dependent</a>. This instantiation is not in an immediate context for the purposes of <a href="sfinae.html" title="cpp/language/sfinae">SFINAE</a>.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span>
<span class="kw4">auto</span> f<span class="br0">(</span>T t<span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> t<span class="sy4">;</span> <span class="br0">}</span>
<span class="kw1">typedef</span> decltype<span class="br0">(</span>f<span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span><span class="br0">)</span> fint_t<span class="sy4">;</span>    <span class="co1">// instantiates f&lt;int&gt; to deduce return type</span>
 
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span>
<span class="kw4">auto</span> f<span class="br0">(</span>T<span class="sy2">*</span> t<span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="sy2">*</span>t<span class="sy4">;</span> <span class="br0">}</span>
<span class="kw4">void</span> g<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw4">int</span> <span class="br0">(</span><span class="sy2">*</span>p<span class="br0">)</span><span class="br0">(</span><span class="kw4">int</span><span class="sy2">*</span><span class="br0">)</span> <span class="sy1">=</span> <span class="sy3">&amp;</span>f<span class="sy4">;</span> <span class="br0">}</span> <span class="co1">// instantiates both fs to determine return types,</span>
                                  <span class="co1">// chooses second template overload</span></pre></div></div>
<p>Redeclarations or specializations of functions or function templates that use return type deduction must use the same return type placeholders:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">auto</span> f<span class="br0">(</span><span class="kw4">int</span> num<span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> num<span class="sy4">;</span> <span class="br0">}</span>
<span class="co1">// int f(int num);            // error: no placeholder return type</span>
<span class="co1">// decltype(auto) f(int num); // error: different placeholder</span>
 
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">typename</span> T<span class="sy1">&gt;</span>
<span class="kw4">auto</span> g<span class="br0">(</span>T t<span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> t<span class="sy4">;</span> <span class="br0">}</span>
<span class="kw1">template</span> <span class="kw4">auto</span> g<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span>     <span class="co1">// okay: return type is int</span>
<span class="co1">// template char g(char); // error: not a specialization of the primary template g</span></pre></div></div>
<p>Similarly, redeclarations or specializations of functions or function templates that do not use return type deduction must not use a placeholder:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">int</span> num<span class="br0">)</span><span class="sy4">;</span>
<span class="co1">// auto f(int num) { return num; } // error: not a redeclaration of f</span>
 
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">typename</span> T<span class="sy1">&gt;</span>
T g<span class="br0">(</span>T t<span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> t<span class="sy4">;</span> <span class="br0">}</span>
<span class="kw1">template</span> <span class="kw4">int</span> g<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span>      <span class="co1">// okay: specialize T as int</span>
<span class="co1">// template auto g(char); // error: not a specialization of the primary template g</span></pre></div></div>
<p><a href="function_template.html#Explicit_instantiation" title="cpp/language/function template">Explicit instantiation declarations</a> do not themselves instantiate function templates that use return type deduction:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">typename</span> T<span class="sy1">&gt;</span>
<span class="kw4">auto</span> f<span class="br0">(</span>T t<span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> t<span class="sy4">;</span> <span class="br0">}</span>
<span class="kw4">extern</span> <span class="kw1">template</span> <span class="kw4">auto</span> f<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// does not instantiate f&lt;int&gt;</span>
 
<span class="kw4">int</span> <span class="br0">(</span><span class="sy2">*</span>p<span class="br0">)</span><span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span> <span class="sy1">=</span> f<span class="sy4">;</span> <span class="co1">// instantiates f&lt;int&gt; to determine its return type,</span>
                   <span class="co1">// but an explicit instantiation definition </span>
                   <span class="co1">// is still required somewhere in the program</span></pre></div></div>
</td>
<td><span class="t-mark-rev t-since-cxx14">(since C++14)</span></td></tr>
</table>
<h3><span class="mw-headline" id="Parameter_list">Parameter list</span></h3>
<p>Parameter list determines the arguments that can be specified when the function is called. It is a comma-separated list of <i>parameter declarations</i>, each of which has the following syntax:
</p>
<table class="t-sdsc-begin">

<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">attr</span><span class="t-mark">(optional)</span> <span class="t-spar">decl-specifier-seq</span> <span class="t-spar">declarator</span>
</td>
<td> (1)
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">attr</span><span class="t-mark">(optional)</span> <span class="t-spar">decl-specifier-seq</span> <span class="t-spar">declarator</span> <code><b>= </b></code> <span class="t-spar">initializer</span>
</td>
<td> (2)
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">attr</span><span class="t-mark">(optional)</span> <span class="t-spar">decl-specifier-seq</span> <span class="t-spar">abstract-declarator</span><span class="t-mark">(optional)</span>
</td>
<td> (3)
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">attr</span><span class="t-mark">(optional)</span> <span class="t-spar">decl-specifier-seq</span> <span class="t-spar">abstract-declarator</span><span class="t-mark">(optional)</span> <code><b>= </b></code> <span class="t-spar">initializer</span>
</td>
<td> (4)
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <code><b>void</b></code>
</td>
<td> (5)
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<div class="t-li1"><span class="t-li">1)</span> Declares a named (formal) parameter. For the meanings of <span class="t-spar">decl-specifier-seq</span> and <span class="t-spar">declarator</span>, see <a href="declarations.html" title="cpp/language/declarations">declarations</a>.
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">int</span> a, <span class="kw4">int</span><span class="sy2">*</span> p, <span class="kw4">int</span> <span class="br0">(</span><span class="sy2">*</span><span class="br0">(</span><span class="sy2">*</span>x<span class="br0">)</span><span class="br0">(</span><span class="kw4">double</span><span class="br0">)</span><span class="br0">)</span><span class="br0">[</span><span class="nu0">3</span><span class="br0">]</span><span class="br0">)</span><span class="sy4">;</span></pre></div></div></div>
<div class="t-li1"><span class="t-li">2)</span> Declares a named (formal) parameter with a <a href="default_arguments.html" title="cpp/language/default arguments">default value</a>.
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">int</span> a <span class="sy1">=</span> <span class="nu0">7</span>, <span class="kw4">int</span><span class="sy2">*</span> p <span class="sy1">=</span> nullptr, <span class="kw4">int</span> <span class="br0">(</span><span class="sy2">*</span><span class="br0">(</span><span class="sy2">*</span>x<span class="br0">)</span><span class="br0">(</span><span class="kw4">double</span><span class="br0">)</span><span class="br0">)</span><span class="br0">[</span><span class="nu0">3</span><span class="br0">]</span> <span class="sy1">=</span> nullptr<span class="br0">)</span><span class="sy4">;</span></pre></div></div></div>
<div class="t-li1"><span class="t-li">3)</span> Declares an unnamed parameter.
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">int</span>, <span class="kw4">int</span><span class="sy2">*</span>, <span class="kw4">int</span> <span class="br0">(</span><span class="sy2">*</span><span class="br0">(</span><span class="sy2">*</span><span class="br0">)</span><span class="br0">(</span><span class="kw4">double</span><span class="br0">)</span><span class="br0">)</span><span class="br0">[</span><span class="nu0">3</span><span class="br0">]</span><span class="br0">)</span><span class="sy4">;</span></pre></div></div></div>
<div class="t-li1"><span class="t-li">4)</span> Declares an unnamed parameter with a <a href="default_arguments.html" title="cpp/language/default arguments">default value</a>.
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">int</span> <span class="sy1">=</span> <span class="nu0">7</span>, <span class="kw4">int</span><span class="sy2">*</span> <span class="sy1">=</span> nullptr, <span class="kw4">int</span> <span class="br0">(</span><span class="sy2">*</span><span class="br0">(</span><span class="sy2">*</span><span class="br0">)</span><span class="br0">(</span><span class="kw4">double</span><span class="br0">)</span><span class="br0">)</span><span class="br0">[</span><span class="nu0">3</span><span class="br0">]</span> <span class="sy1">=</span> nullptr<span class="br0">)</span><span class="sy4">;</span></pre></div></div></div>
<div class="t-li1"><span class="t-li">5)</span> Indicates that the function takes no parameters, it is the exact synonym for an empty parameter list: <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span><span class="sy4">;</span></span></span> and <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">int</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span></span></span> declare the same function. Note that the type <code>void</code> (possibly cv-qualified) cannot be used in a parameter list otherwise: <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">void</span>, <span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span></span></span>  and <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">const</span> <span class="kw4">void</span><span class="br0">)</span><span class="sy4">;</span></span></span> are errors (although derived types, such as <code>void*</code> can be used). In a template, only non-dependent void type can be used (a function taking a single parameter of type <code>T</code> does not become a no-parameter function if instantiated with <code>T = void</code>).</div>
<p>An ellipsis <code><b>...</b></code> may appear at the end of the parameter list; this declares a <a href="variadic_arguments.html" title="cpp/language/variadic arguments">variadic function</a>:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">int</span> printf<span class="br0">(</span><span class="kw4">const</span> <span class="kw4">char</span><span class="sy2">*</span> fmt ...<span class="br0">)</span><span class="sy4">;</span></pre></div></div>
<p>For compatibility with C89, an optional comma may appear before the ellipsis if the parameter list contains at least one parameter:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">int</span> printf<span class="br0">(</span><span class="kw4">const</span> <span class="kw4">char</span><span class="sy2">*</span> fmt, ...<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK, same as above</span></pre></div></div>
 <table class="t-rev-begin">
<tr class="t-rev t-until-cxx17"><td>
<p>Although <span class="t-spar">decl-specifier-seq</span> implies there can exist <a href="declarations.html#Specifiers" title="cpp/language/declarations">specifiers</a> other than type specifiers, the only other specifier allowed is <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">register</span></span></span> <span class="t-rev-inl t-until-cxx11"><span>as well as <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">auto</span></span></span></span> <span><span class="t-mark-rev t-until-cxx11">(until C++11)</span></span></span>, and it has no effect.
</p>
</td>
<td><span class="t-mark-rev t-until-cxx17">(until C++17)</span></td></tr>
</table>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx20"><td>
<p>If any of the function parameters uses a <i>placeholder</i> (either <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">auto</span></span></span> or a <a href="../concepts.html" title="cpp/concepts">concept type</a>), the function declaration is instead an <a href="function_template.html#Abbreviated_function_template" title="cpp/language/function template">abbreviated function template</a> declaration:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">void</span> f1<span class="br0">(</span><span class="kw4">auto</span><span class="br0">)</span><span class="sy4">;</span>    <span class="co1">// same as template&lt;class T&gt; void f(T)</span>
<span class="kw4">void</span> f2<span class="br0">(</span>C1 <span class="kw4">auto</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// same as template&lt;C1 T&gt; void f7(T), if C1 is a concept</span></pre></div></div>
</td>
<td><span class="t-mark-rev t-since-cxx20">(since C++20)</span></td></tr>
</table>
<p>Parameter names declared in function declarations are usually for only self-documenting purposes. They are used (but remain optional) in function definitions.
</p><p>The type of each function parameter in the parameter list is determined according to the following rules:
</p>
<div class="t-li1"><span class="t-li">1)</span> First, <span class="t-spar">decl-specifier-seq</span> and the declarator are combined as in any <a href="declarations.html" title="cpp/language/declarations">declaration</a> to determine the type.</div>
<div class="t-li1"><span class="t-li">2)</span> If the type is "array of T" or "array of unknown bound of T", it is replaced by the type "pointer to T"</div>
<div class="t-li1"><span class="t-li">3)</span> If the type is a function type F, it is replaced by the type "pointer to F"</div>
<div class="t-li1"><span class="t-li">4)</span> Top-level cv-qualifiers are dropped from the parameter type (This adjustment only affects the function type, but doesn't modify the property of the parameter: <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">const</span> <span class="kw4">int</span> p, decltype<span class="br0">(</span>p<span class="br0">)</span><span class="sy2">*</span><span class="br0">)</span><span class="sy4">;</span></span></span> and <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">int</span>, <span class="kw4">const</span> <span class="kw4">int</span><span class="sy2">*</span><span class="br0">)</span><span class="sy4">;</span></span></span> declare the same function) </div>
<p>Because of these rules, the following function declarations declare exactly the same function:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">char</span> s<span class="br0">[</span><span class="nu0">3</span><span class="br0">]</span><span class="br0">)</span><span class="sy4">;</span>
<span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">char</span><span class="br0">[</span><span class="br0">]</span><span class="br0">)</span><span class="sy4">;</span>
<span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">char</span><span class="sy2">*</span> s<span class="br0">)</span><span class="sy4">;</span>
<span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">char</span><span class="sy2">*</span> <span class="kw4">const</span><span class="br0">)</span><span class="sy4">;</span>
<span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">char</span><span class="sy2">*</span> <span class="kw4">volatile</span> s<span class="br0">)</span><span class="sy4">;</span></pre></div></div>
<p>The following declarations also declare exactly the same function:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">int</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
<span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">int</span> <span class="br0">(</span><span class="sy2">*</span>g<span class="br0">)</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span></pre></div></div>
<p>An ambiguity arises in a parameter list when a type name is nested in parentheses <span class="t-rev-inl t-since-cxx11"><span>(including <a href="lambda.html" title="cpp/language/lambda">lambda expressions</a>)</span> <span><span class="t-mark-rev t-since-cxx11">(since C++11)</span></span></span>. In this case, the choice is between the declaration of a parameter of type pointer to function and the declaration of a parameter with redundant parentheses around the identifier of the <span class="t-spar">declarator</span>. The resolution is to consider the type name as a <a href="declarations.html#Specifiers" title="cpp/language/declarations">simple type specifier</a> (which is the pointer to function type):
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">class</span> C <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">int</span><span class="br0">(</span>C<span class="br0">)</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span> <span class="co1">// void f(int(*fp)(C param)) {}</span>
                  <span class="co1">// NOT void f(int C) {}</span>
 
<span class="kw4">void</span> g<span class="br0">(</span><span class="kw4">int</span> <span class="sy2">*</span><span class="br0">(</span>C<span class="br0">[</span><span class="nu0">10</span><span class="br0">]</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// void g(int *(*fp)(C param[10]));</span>
                      <span class="co1">// NOT void g(int *C[10]);</span></pre></div></div>
<p>Parameter type cannot be a type that includes a reference or a pointer to array of unknown bound, including a multi-level pointers/arrays of such types, or a pointer to functions whose parameters are such types.
<span id="six_dots"></span>
</p>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx11"><td>
<p>The ellipsis that indicates <a href="variadic_arguments.html" title="cpp/language/variadic arguments">variadic arguments</a> need not be preceded by a comma, even if it follows the ellipsis that indicates a <a href="parameter_pack.html" title="cpp/language/parameter pack">parameter pack</a> expansion, so the following function templates are exactly the same:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">typename</span>... <span class="me1">Args</span><span class="sy1">&gt;</span>
<span class="kw4">void</span> f<span class="br0">(</span>Args..., ...<span class="br0">)</span><span class="sy4">;</span>
 
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">typename</span>... <span class="me1">Args</span><span class="sy1">&gt;</span>
<span class="kw4">void</span> f<span class="br0">(</span>Args... ...<span class="br0">)</span><span class="sy4">;</span>
 
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">typename</span>... <span class="me1">Args</span><span class="sy1">&gt;</span>
<span class="kw4">void</span> f<span class="br0">(</span>Args......<span class="br0">)</span><span class="sy4">;</span></pre></div></div>
<p>An example of when such declaration might be used is the <a href="../types/is_function.html#Possible_implementation" title="cpp/types/is function">possible implementation</a> of <span class="t-lc"><a href="../types/is_function.html" title="cpp/types/is function">std::is_function</a></span>.
</p>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;cstdio&gt;</span>
 
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">typename</span>... <span class="me1">Variadic</span>, <span class="kw1">typename</span>... <span class="me1">Args</span><span class="sy1">&gt;</span>
<span class="kw4">constexpr</span> <span class="kw4">void</span> invoke<span class="br0">(</span><span class="kw4">auto</span> <span class="br0">(</span><span class="sy2">*</span>fun<span class="br0">)</span><span class="br0">(</span>Variadic......<span class="br0">)</span>, Args... <span class="me1">args</span><span class="br0">)</span>
<span class="br0">{</span>
    fun<span class="br0">(</span>args...<span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    invoke<span class="br0">(</span><a href="../io/c/fprintf.html"><span class="kw1849">std::<span class="me2">printf</span></span></a>, <span class="st0">"%dm•%dm•%dm = %d%s%c"</span>, <span class="nu0">2</span>,<span class="nu0">3</span>,<span class="nu0">7</span>, <span class="nu0">2</span><span class="sy2">*</span><span class="nu0">3</span><span class="sy2">*</span><span class="nu0">7</span>, <span class="st0">"m³"</span>, <span class="st0">'<span class="es1">\n</span>'</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p>Output:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">2m•3m•7m = 42m³</pre></div></div> 
</div>
</td>
<td><span class="t-mark-rev t-since-cxx11">(since C++11)</span></td></tr>
</table>
<h3><span class="mw-headline" id="Function_definition">Function definition</span></h3>
<p>A non-member function definition may appear at namespace scope only (there are no nested functions). A <a href="member_functions.html" title="cpp/language/member functions">member function</a> definition may also appear in the body of a <a href="class.html" title="cpp/language/class">class definition</a>. They have the following syntax:
</p>
<table class="t-sdsc-begin">

<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td class="t-sdsc-nopad"> <span class="t-spar">attr</span><span class="t-mark">(optional)</span> <span class="t-spar">decl-specifier-seq</span><span class="t-mark">(optional)</span> <span class="t-spar">declarator</span> <span class="t-spar">virt-specifier-seq</span><span class="t-mark">(optional)</span> <span class="t-spar">function-body</span>
</td>
<td class="t-sdsc-nopad">
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<p>where <span class="t-spar">function-body</span> is one of the following
</p>
<table class="t-sdsc-begin">

<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">ctor-initializer</span><span class="t-mark">(optional)</span> <span class="t-spar">compound-statement</span>
</td>
<td> (1)
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">function-try-block</span>
</td>
<td> (2)
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <code><b>=</b></code> <code><b>delete</b></code> <code><b>;</b></code>
</td>
<td> (3)
</td>
<td> <span class="t-mark-rev t-since-cxx11">(since C++11)</span>
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <code><b>=</b></code> <code><b>default</b></code> <code><b>;</b></code>
</td>
<td> (4)
</td>
<td> <span class="t-mark-rev t-since-cxx11">(since C++11)</span>
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<div class="t-li1"><span class="t-li">1)</span> regular function body</div>
<div class="t-li1"><span class="t-li">2)</span> <a href="function-try-block.html" title="cpp/language/function-try-block">function-try-block</a> (which is a regular function body wrapped in a try/catch block)</div>
<div class="t-li1"><span class="t-li">3)</span> explicitly deleted function definition</div>
<div class="t-li1"><span class="t-li">4)</span> explicitly defaulted function definition, only allowed for <a href="member_functions.html#Special_member_functions" title="cpp/language/member functions">special member functions</a><span class="t-rev-inl t-since-cxx20"><span> and <a href="default_comparisons.html" title="cpp/language/default comparisons">comparison operator functions</a> </span> <span><span class="t-mark-rev t-since-cxx20">(since C++20)</span></span></span></div>
<table class="t-par-begin">


<tr class="t-par">
<td> <span class="t-spar">attr</span>
</td>
<td> -
</td>
<td> <span class="t-mark-rev t-since-cxx11">(since C++11)</span> optional list of <a href="attributes.html" title="cpp/language/attributes">attributes</a>. These attributes are combined with the attributes after the identifier in the <span class="t-spar">declarator</span> (see top of this page), if any.
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">decl-specifier-seq</span>
</td>
<td> -
</td>
<td> the return type with specifiers, as in the <a href="declarations.html" title="cpp/language/declarations">declaration grammar</a>
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">declarator</span>
</td>
<td> -
</td>
<td> function declarator, same as in the function declaration grammar above (can be parenthesized). <span class="t-rev-inl t-since-cxx20"><span>as with function declaration, it may be followed by a <span class="t-spar">requires-clause</span></span> <span><span class="t-mark-rev t-since-cxx20">(since C++20)</span></span></span>
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">virt-specifier-seq</span>
</td>
<td> -
</td>
<td> <span class="t-mark-rev t-since-cxx11">(since C++11)</span> <a href="override.html" title="cpp/language/override"><code>override</code></a>, <a href="final.html" title="cpp/language/final"><code>final</code></a>, or their combination in any order (only allowed for non-static member functions)
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">ctor-initializer</span>
</td>
<td> -
</td>
<td> <a href="initializer_list.html" title="cpp/language/initializer list" class="mw-redirect">member initializer list</a>, only allowed in constructors
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">compound-statement</span>
</td>
<td> -
</td>
<td> the brace-enclosed <a href="statements.html#Compound_statements" title="cpp/language/statements">sequence of statements</a> that constitutes the body of a function
</td></tr></table>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">int</span> max<span class="br0">(</span><span class="kw4">int</span> a, <span class="kw4">int</span> b, <span class="kw4">int</span> c<span class="br0">)</span>
<span class="br0">{</span>
    <span class="kw4">int</span> m <span class="sy1">=</span> <span class="br0">(</span>a <span class="sy1">&gt;</span> b<span class="br0">)</span> <span class="sy4">?</span> a <span class="sy4">:</span> b<span class="sy4">;</span>
    <span class="kw1">return</span> <span class="br0">(</span>m <span class="sy1">&gt;</span> c<span class="br0">)</span> <span class="sy4">?</span> m <span class="sy4">:</span> c<span class="sy4">;</span>
<span class="br0">}</span>
 
<span class="co1">// decl-specifier-seq is "int"</span>
<span class="co1">// declarator is "max(int a, int b, int c)"</span>
<span class="co1">// body is { ... }</span></pre></div></div>
<p>The function body is a <a href="statements.html#Compound_statements" title="cpp/language/statements">compound statement</a> (sequence of zero or more statements surrounded by a pair of curly braces), which is executed when the function call is made.
</p><p>The parameter types, as well as the return type of a function definition cannot be (possibly cv-qualified) <a href="incomplete_type.html" title="cpp/language/incomplete type" class="mw-redirect">incomplete</a> <a href="class.html" title="cpp/language/class">class types</a><span class="t-rev-inl t-since-cxx11"><span> unless the function is defined as deleted</span> <span><span class="t-mark-rev t-since-cxx11">(since C++11)</span></span></span>. The completeness check is only made in the function body, which allows <a href="member_functions.html" title="cpp/language/member functions">member functions</a> to return the class in which they are defined (or its enclosing class), even if it is incomplete at the point of definition (it is complete in the function body).
</p><p>The parameters declared in the <span class="t-spar">declarator</span> of a function definition are <a href="scope.html" title="cpp/language/scope">in scope</a> within the body. If a parameter is not used in the function body, it does not need to be named (it's sufficient to use an abstract declarator):
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">void</span> print<span class="br0">(</span><span class="kw4">int</span> a, <span class="kw4">int</span><span class="br0">)</span> <span class="co1">// second parameter is not used</span>
<span class="br0">{</span>
    <a href="../io/c/fprintf.html"><span class="kw1849">std::<span class="me2">printf</span></span></a><span class="br0">(</span><span class="st0">"a = %d<span class="es1">\n</span>"</span>, a<span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p>Even though top-level <a href="cv.html" title="cpp/language/cv">cv-qualifiers</a> on the parameters are discarded in function declarations, they modify the type of the parameter as visible in the body of a function:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">const</span> <span class="kw4">int</span> n<span class="br0">)</span> <span class="co1">// declares function of type void(int)</span>
<span class="br0">{</span>
    <span class="co1">// but in the body, the type of n is const int</span>
<span class="br0">}</span></pre></div></div>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx11"><td>
<h3> <span class="mw-headline" id="Deleted_functions">Deleted functions</span></h3>
<p>If, instead of a function body, the special syntax <code><b>= delete ;</b></code> is used, the function is defined as <i>deleted</i>. Any use of a deleted function is ill-formed (the program will not compile). This includes calls, both explicit (with a function call operator) and implicit (a call to deleted overloaded operator, special member function, allocation function etc), constructing a pointer or pointer-to-member to a deleted function, and even the use of a deleted function in an unevaluated expression. However, implicit <a href="definition.html#ODR-use" title="cpp/language/definition">ODR-use</a> of a non-pure virtual member function that happens to be deleted is allowed.
</p><p>If the function is overloaded, <a href="overload_resolution.html" title="cpp/language/overload resolution">overload resolution</a> takes place first, and the program is only ill-formed if the deleted function was selected:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> sometype
<span class="br0">{</span>
    <span class="kw4">void</span><span class="sy2">*</span> <a href="../memory/new/operator_new.html"><span class="kw689">operator new</span></a><span class="br0">(</span><a href="../types/size_t.html"><span class="kw106">std::<span class="me2">size_t</span></span></a><span class="br0">)</span> <span class="sy1">=</span> delete<span class="sy4">;</span>
    <span class="kw4">void</span><span class="sy2">*</span> <a href="../memory/new/operator_new.html"><span class="kw689">operator new</span></a><span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><a href="../types/size_t.html"><span class="kw106">std::<span class="me2">size_t</span></span></a><span class="br0">)</span> <span class="sy1">=</span> delete<span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
sometype<span class="sy2">*</span> p <span class="sy1">=</span> new sometype<span class="sy4">;</span> <span class="co1">// error: attempts to call deleted sometype::operator new</span></pre></div></div>
<p>The deleted definition of a function must be the first declaration in a translation unit: a previously-declared function cannot be redeclared as deleted:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> sometype <span class="br0">{</span> sometype<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
sometype<span class="sy4">::</span><span class="me2">sometype</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">=</span> delete<span class="sy4">;</span> <span class="co1">// error: must be deleted on the first declaration</span></pre></div></div>
<h3> <span class="mw-headline" id="User-provided_functions">User-provided functions</span></h3>
<p>A function is <i>user-provided</i> if it is user-declared and not explicitly defaulted or deleted on its first declaration. A user-provided explicitly-defaulted function (i.e., explicitly defaulted after its first declaration) is defined at the point where it is explicitly defaulted; if such a function is implicitly defined as deleted, the program is ill-formed. Declaring a function as defaulted after its first declaration can provide efficient execution and concise definition while enabling a stable binary interface to an evolving code base.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co1">// All special member functions of `trivial` are</span>
<span class="co1">// defaulted on their first declarations respectively,</span>
<span class="co1">// they are not user-provided</span>
<span class="kw1">struct</span> trivial
<span class="br0">{</span>
    trivial<span class="br0">(</span><span class="br0">)</span> <span class="sy1">=</span> <span class="kw1">default</span><span class="sy4">;</span>
    trivial<span class="br0">(</span><span class="kw4">const</span> trivial<span class="sy3">&amp;</span><span class="br0">)</span> <span class="sy1">=</span> <span class="kw1">default</span><span class="sy4">;</span>
    trivial<span class="br0">(</span>trivial<span class="sy3">&amp;&amp;</span><span class="br0">)</span> <span class="sy1">=</span> <span class="kw1">default</span><span class="sy4">;</span>
    trivial<span class="sy3">&amp;</span> operator<span class="sy1">=</span><span class="br0">(</span><span class="kw4">const</span> trivial<span class="sy3">&amp;</span><span class="br0">)</span> <span class="sy1">=</span> <span class="kw1">default</span><span class="sy4">;</span>
    trivial<span class="sy3">&amp;</span> operator<span class="sy1">=</span><span class="br0">(</span>trivial<span class="sy3">&amp;&amp;</span><span class="br0">)</span> <span class="sy1">=</span> <span class="kw1">default</span><span class="sy4">;</span>
    ~trivial<span class="br0">(</span><span class="br0">)</span> <span class="sy1">=</span> <span class="kw1">default</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">struct</span> nontrivial
<span class="br0">{</span>
    nontrivial<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// first declaration</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="co1">// not defaulted on the first declaration,</span>
<span class="co1">// it is user-provided and is defined here</span>
nontrivial<span class="sy4">::</span><span class="me2">nontrivial</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">=</span> <span class="kw1">default</span><span class="sy4">;</span></pre></div></div>
<p><span id="func"></span>
</p>
<h3> <span class="mw-headline" id="func">__func__</span></h3>
<p>Within the function body, the function-local predefined variable <span class="t-lc">__func__</span> is defined as if by
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">static</span> <span class="kw4">const</span> <span class="kw4">char</span> __func__<span class="br0">[</span><span class="br0">]</span> <span class="sy1">=</span> <span class="st0">"function-name"</span><span class="sy4">;</span></pre></div></div>
<p>This variable has block scope and static storage duration:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> S
<span class="br0">{</span>
    S<span class="br0">(</span><span class="br0">)</span><span class="sy4">:</span> s<span class="br0">(</span>__func__<span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span> <span class="co1">// okay: initializer-list is part of function body</span>
    <span class="kw4">const</span> <span class="kw4">char</span><span class="sy2">*</span> s<span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">const</span> <span class="kw4">char</span><span class="sy2">*</span> s <span class="sy1">=</span> __func__<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// error: parameter-list is part of declarator</span></pre></div></div>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;iostream&gt;</span>
 
<span class="kw4">void</span> Foo<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <a href="../io/cout.html"><span class="kw1762">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> __func__ <span class="sy1">&lt;&lt;</span> <span class="st0">' '</span><span class="sy4">;</span> <span class="br0">}</span>
 
<span class="kw1">struct</span> Bar
<span class="br0">{</span>
    Bar<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <a href="../io/cout.html"><span class="kw1762">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> __func__ <span class="sy1">&lt;&lt;</span> <span class="st0">' '</span><span class="sy4">;</span> <span class="br0">}</span>
    ~Bar<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <a href="../io/cout.html"><span class="kw1762">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> __func__ <span class="sy1">&lt;&lt;</span> <span class="st0">' '</span><span class="sy4">;</span> <span class="br0">}</span>
    <span class="kw1">struct</span> Pub <span class="br0">{</span> Pub<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <a href="../io/cout.html"><span class="kw1762">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> __func__ <span class="sy1">&lt;&lt;</span> <span class="st0">' '</span><span class="sy4">;</span> <span class="br0">}</span> <span class="br0">}</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    Foo<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
    Bar bar<span class="sy4">;</span>
    Bar<span class="sy4">::</span><span class="me2">Pub</span> pub<span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p>Possible output:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">Foo Bar Pub ~Bar</pre></div></div> 
</div>
</td>
<td><span class="t-mark-rev t-since-cxx11">(since C++11)</span></td></tr>
</table>
<h3><span class="mw-headline" id="Notes">Notes</span></h3>
<p>In case of ambiguity between a variable declaration using the direct-initialization syntax and a function declaration, the compiler always chooses function declaration; see <a href="direct_initialization.html#Notes" title="cpp/language/direct initialization">direct-initialization</a>.
</p>
<h3><span class="mw-headline" id="Example">Example</span></h3>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;iostream&gt;</span>
<span class="co2">#include &lt;string&gt;</span>
 
<span class="co1">// simple function with a default argument, returning nothing</span>
<span class="kw4">void</span> f0<span class="br0">(</span><span class="kw4">const</span> <a href="../string/basic_string.html"><span class="kw1233">std::<span class="me2">string</span></span></a><span class="sy3">&amp;</span> arg <span class="sy1">=</span> <span class="st0">"world!"</span><span class="br0">)</span>
<span class="br0">{</span>
    <a href="../io/cout.html"><span class="kw1762">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"Hello, "</span> <span class="sy1">&lt;&lt;</span> arg <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
<span class="br0">}</span>
 
<span class="co1">// the declaration is in namespace (file) scope</span>
<span class="co1">// (the definition is provided later)</span>
<span class="kw4">int</span> f1<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
 
<span class="co1">// function returning a pointer to f0, pre-C++11 style</span>
<span class="kw4">void</span> <span class="br0">(</span><span class="sy2">*</span>fp03<span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="br0">(</span><span class="kw4">const</span> <a href="../string/basic_string.html"><span class="kw1233">std::<span class="me2">string</span></span></a><span class="sy3">&amp;</span><span class="br0">)</span>
<span class="br0">{</span>
    <span class="kw1">return</span> f0<span class="sy4">;</span>
<span class="br0">}</span>
 
<span class="co1">// function returning a pointer to f0, with C++11 trailing return type</span>
<span class="kw4">auto</span> fp11<span class="br0">(</span><span class="br0">)</span> <span class="sy2">-</span><span class="sy1">&gt;</span> <span class="kw4">void</span><span class="br0">(</span><span class="sy2">*</span><span class="br0">)</span><span class="br0">(</span><span class="kw4">const</span> <a href="../string/basic_string.html"><span class="kw1233">std::<span class="me2">string</span></span></a><span class="sy3">&amp;</span><span class="br0">)</span>
<span class="br0">{</span>
    <span class="kw1">return</span> f0<span class="sy4">;</span>
<span class="br0">}</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    f0<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
    fp03<span class="br0">(</span><span class="br0">)</span><span class="br0">(</span><span class="st0">"test!"</span><span class="br0">)</span><span class="sy4">;</span>
    fp11<span class="br0">(</span><span class="br0">)</span><span class="br0">(</span><span class="st0">"again!"</span><span class="br0">)</span><span class="sy4">;</span>
    <span class="kw4">int</span> f2<span class="br0">(</span><a href="../string/basic_string.html"><span class="kw1233">std::<span class="me2">string</span></span></a><span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span> <span class="co1">// declaration in function scope</span>
    <a href="../io/cout.html"><span class="kw1762">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"f2(<span class="es1">\"</span>bad<span class="es1">\"</span>): "</span> <span class="sy1">&lt;&lt;</span> f2<span class="br0">(</span><span class="st0">"bad"</span><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
    <a href="../io/cout.html"><span class="kw1762">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"f2(<span class="es1">\"</span>42<span class="es1">\"</span>): "</span> <span class="sy1">&lt;&lt;</span> f2<span class="br0">(</span><span class="st0">"42"</span><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
<span class="br0">}</span>
 
<span class="co1">// simple non-member function returning int</span>
<span class="kw4">int</span> f1<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    <span class="kw1">return</span> <span class="nu8">007</span><span class="sy4">;</span>
<span class="br0">}</span>
 
<span class="co1">// function with an exception specification and a function try block</span>
<span class="kw4">int</span> f2<span class="br0">(</span><a href="../string/basic_string.html"><span class="kw1233">std::<span class="me2">string</span></span></a> str<span class="br0">)</span> <span class="kw1">noexcept</span>
<span class="kw1">try</span>
<span class="br0">{</span>
    <span class="kw1">return</span> <a href="../string/basic_string/stol.html"><span class="kw1260">std::<span class="me2">stoi</span></span></a><span class="br0">(</span>str<span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span>
<span class="kw1">catch</span> <span class="br0">(</span><span class="kw4">const</span> <a href="../error/exception.html"><span class="kw765">std::<span class="me2">exception</span></span></a><span class="sy3">&amp;</span> e<span class="br0">)</span>
<span class="br0">{</span>
    <a href="../io/cerr.html"><span class="kw1764">std::<span class="me2">cerr</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"stoi() failed!<span class="es1">\n</span>"</span><span class="sy4">;</span>
    <span class="kw1">return</span> <span class="nu0">0</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p>Possible output:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">stoi() failed!
Hello, world!
Hello, test!
Hello, again!
f2("bad"): 0
f2("42"): 42</pre></div></div> 
</div>
<h3><span class="mw-headline" id="Defect_reports">Defect reports</span></h3>
<p>The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
</p>
<table class="dsctable" style="font-size:0.8em">
<tr>
<th> DR
</th>
<th> Applied to
</th>
<th> Behavior as published
</th>
<th> Correct behavior
</th></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/135.html">CWG 135</a>
</td>
<td> C++98
</td>
<td> member functions defined in class<br>could not have a parameter of or return<br>its own class because it is incomplete
</td>
<td> allowed
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/393.html">CWG 393</a>
</td>
<td> C++98
</td>
<td> types that include pointers/references to<br>array of unknown bound could not be parameters
</td>
<td> such types are allowed
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/452.html">CWG 452</a>
</td>
<td> C++98
</td>
<td> member initializer list was not a part of function body
</td>
<td> made it a part of function<br>body by modifying the syntax<br>of function definition
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/577.html">CWG 577</a>
</td>
<td> C++98
</td>
<td> 1. dependent type <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span></span></span> could be used<br>to declare a function taking no parameters<br>2. a parameter could have cv-qualified <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span></span></span> type
</td>
<td> 1. only non-dependent<br><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span></span></span> is allowed<br>2. cv-qualified <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span></span></span> types<br>    are also prohibited
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/1327.html">CWG 1327</a>
</td>
<td> C++11
</td>
<td> defaulted or deleted functions could not<br>be specified with <span class="t-c"><span class="mw-geshi cpp source-cpp">override</span></span> or <span class="t-c"><span class="mw-geshi cpp source-cpp">final</span></span>
</td>
<td> allowed
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/1355.html">CWG 1355</a>
</td>
<td> C++11
</td>
<td> only special member functions could be user-provided
</td>
<td> extended to all functions
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/1394.html">CWG 1394</a>
</td>
<td> C++11
</td>
<td> deleted functions could not have any parameter of<br>an incomplete type or return an incomplete type
</td>
<td> incomplete type allowed
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/1824.html">CWG 1824</a>
</td>
<td> C++98
</td>
<td> the completeness check on parameter type and<br>return type of a function definition could be made<br>outside the context of the function definition
</td>
<td> only make the completeness<br>check in the context of<br>the function definition
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/1877.html">CWG 1877</a>
</td>
<td> C++14
</td>
<td> return type deduction treated <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">return</span><span class="sy4">;</span></span></span> as <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">return</span> <span class="kw4">void</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span></span></span>
</td>
<td> simply deduce the return<br>type as <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span></span></span> in this case
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/2015.html">CWG 2015</a>
</td>
<td> C++11
</td>
<td> the implicit odr-use of a deleted<br>virtual function was ill-formed
</td>
<td> such odr-uses are exempt<br>from the use prohibition
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/2044.html">CWG 2044</a>
</td>
<td> C++14
</td>
<td> return type deduction on functions returning <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span></span></span><br>would fail if the declared return type is <span class="t-c"><span class="mw-geshi cpp source-cpp">decltype<span class="br0">(</span><span class="kw4">auto</span><span class="br0">)</span></span></span>
</td>
<td> updated the deduction<br>rule to handle this case
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/2081.html">CWG 2081</a>
</td>
<td> C++14
</td>
<td> function redeclarations could use return type<br>deduction even if the initial declaration does not
</td>
<td> not allowed
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/2145.html">CWG 2145</a>
</td>
<td> C++98
</td>
<td> the <span class="t-spar">declarator</span> in function definition could not be parenthesized
</td>
<td> allowed
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/2259.html">CWG 2259</a>
</td>
<td> C++11
</td>
<td> the ambiguity resolution rule regarding parenthesized<br>type names did not cover lambda expressions
</td>
<td> covered
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/2430.html">CWG 2430</a>
</td>
<td> C++98
</td>
<td> in the definition of a member function in a class definition,<br>the type of that class could not be the return type or<br>parameter type due to the resolution of <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/1824.html">CWG issue 1824</a>
</td>
<td> only make the completeness<br>check in the function body
</td></tr></table>
<h3><span class="mw-headline" id="See_also">See also</span></h3>
<table class="t-dsc-begin">

</table>

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Preprocessor generated node count: 15037/1000000
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